Add a parse_from_string function

[macrotex]

I have reason to use Pod::Text with strings rather than files or filehandles. Please add a "parse_from_string" function that would work something like

my $pod_input     = "=pod\n\n=head1 Head1\n\n...";
my $rendered_pod = parse_from_string($pod_input);

[macrotex]

Mistakenly created second duplicate issue.